The continuing pursuit of more fuel efficient motor vehicles is forcing vehicular automatic transmission builders to make transmissions ever more energy efficient. There are a number of types of automatic transmission including stepped automatic transmissions, automated manual transmissions, continuously variable transmissions and dual clutch transmissions. Each type of automatic transmission offers some advantages over the others when used in motor vehicles, however, the ability to reduce size and weight provides a benefit regardless of type. In any automatic transmission where a paper composite, fluid lubricated clutch is used (e.g. stepped automatic transmissions, continuously variable transmissions and dual clutch transmissions), reduction in the size by, for example, reducing the number of plates used in the clutch, will reduce the size and weight of the overall transmission. Increasing the friction level in the clutch has the desirable effect of increasing the level of torque that can be transferred through the clutch which, in turn, requires less surface area to transmit the same amount of torque. Therefore, in a wet clutch having, for example, five fiber composite plates, a 20% increase in dynamic friction provided by the fluid and friction lining would allow for the removal of one paper plate and one steel plate, thereby providing a corresponding 20% decrease in the weight and size of the clutch.
Applicants have now discovered that lubricating fluids, particularly lubricating power transmitting fluids, more particularly automatic transmission fluids, incorporating borated detergents, when used in conjunction with wet friction clutches having composite friction linings having a surface coating of silica based particles, produce wet friction clutch—lubricant systems that deliver increased levels of dynamic friction that enable the transmissions in which they are used to be made smaller, decreasing the size and weight of the transmission and resulting in an improvement in fuel efficiency for the overall vehicle.
U.S. Pat. No. 4,792,410, Schwind et al. discloses the use of a combination of a friction modifier and borated metal detergent where the metal ion is an alkali metal or alkaline earth metal, in lubricants for manual transmissions, and exemplifies the use of overbased borated sodium detergents. The use of the claimed lubricants is to provide reduced double detent and clashing (which relates to metal on metal contact) during manual transmission shifting. Manual transmissions do not contain wet friction clutches. The Schwind et al. patent does not suggest that the selection of the metal ion of the detergent has any effect on performance and does not discuss or contemplate the use of the compositions disclosed therein in automatic transmissions or in conjunction with any other device including wet friction clutches.
U.S. Pat. No. 6,451,745 to Ward discloses lubricants for use in continuously variable transmissions which lubricants contain a borated dispersant and a borated detergent, which lubricants have a boron content of at least 250 ppm. The Ward patent does not describe wet friction clutches having composite friction linings having a surface coating of silica based particles, or suggest that the selection of the borated detergent has any effect on paper based clutch performance, especially in the level of friction generated.